Hello World!

Getting started with Telegraph is as simple as saying "Hello World"

This tutorial will walk you through sending a message to an EVM chain from an EVM chain. Step 1: Create a basic solidity contract To start, let's create a basic solidity contract. This is the foundation of your bridge contract, and we will be building upon it in the next steps.

First, you'll need to define the version of the solidity compiler that you will be using. You can do this by including the pragma statement at the very top of your contract code, like this:

pragma solidity ^0.8.0;

Now, let's define our contract using the contract keyword, and call it HelloWorld.

contract HelloWorld {
    // ...
}

Now that we have the basic structure of our contract, we can add some state variables, functions, and events. For the sake of this example, let's add a string variable called message, that will be publicly accessible. We will also add a constructor function that sets the initial value of the message to "Hello, World!". Lastly, let's create two functions: setMessage that can change the value of the message, and getMessage that will allow us to read the message from the contract.

contract HelloWorld {
    string public message;

    constructor() public {
        message = "Hello, World!";
    }

    // Function to set a new message
    function setMessage(string calldata newMessage) external {
        message = newMessage;
        emit NewMessage(newMessage);
    }

    // Function to get the current message
    function getMessage() external view returns (string memory) {
        return message;
    }
}

That's it! You've successfully created your first solidity contract. This is the foundation of your bridge contract, and now you have a basic structure to build on. In the next steps, we'll be adding more functionality such as allowing the contract to communicate with other chains. So don't worry if it all seems simple for now.

Step 2: Add an interface with the bridge contract's outboundMessage function

Now that we have our basic contract set up, we need to add the ability for it to communicate with other chains. To do this, we will create an interface that defines the outboundMessage function that our contract will use to send messages to the bridge contract.

An interface is a way to define a contract's expected behavior, without specifying its implementation.

Here is an example of an interface for a bridge contract called Bridge that defines the outboundMessage function:

// Define the Bridge interface
    interface Bridge {
        function outboundMessage(
            address from, 
            address to, 
            CrossChainData calldata data, 
            string calldata chain
        ) external payable;
    }

The outboundMessage function accepts several parameters:

  • from is the address of the contract that is sending the message.

  • to is the address of the contract that the message is intended for.

  • addresses, integers, strings, bools are arrays that can be used to pass along data in the message.

  • chain is a string parameter specifying which chain the message is going to.

  • The external payable defines that this function can be called by external contracts and it allows also to make payments as msg.value.

You will then need to add a state variable to your HelloWorld contract that will reference the address of the deployed bridge contract, so that the contract knows where to send the messages.

contract HelloWorld {
    // ...
    Bridge public bridge;
    //...
}

In the constructor of the contract you can set the value of the bridge variable with the address of the deployed bridge contract. Now that we have the Bridge interface defined and the bridge state variable added to our HelloWorld contract, we need to initialize the bridge state variable in the constructor of the HelloWorld contract with the address of the deployed bridge contract.

Here's an example of how you can do this in the constructor function of the HelloWorld contract:

// Set the bridge address upon deployment
    constructor(address bridgeAddress) {
        message = "Hello, World!";
        bridge = Bridge(bridgeAddress);
    }

As you can see, we are passing in the address of the deployed bridge contract as an argument when the HelloWorld contract is deployed and we are assigning that value to the bridge state variable.

Keep in mind that you should always validate the _bridgeAddress to be sure that it is the correct address.

It is important to make sure that the bridge contract is deployed and available on the network before you deploy the HelloWorld contract.

That's it for this step! Your HelloWorld contract is now able to communicate with the bridge contract using the outboundMessage function defined in the Bridge interface.

Step 3: Call the outboundMessage function from the basic solidity contract

Now that our HelloWorld contract is able to communicate with the bridge contract, we can call the outboundMessage function from the basic solidity contract to send messages.

You can do this by defining a new function in your HelloWorld contract, for example, sendMessage, which will be responsible for calling the outboundMessage function of the bridge contract.

Here's an example of how you can define the sendMessage function in the HelloWorld contract:

function sendMessage() public payable returns (bool) {
        require(msg.value > 0, "Fee amount must be greater than 0");
        address[] memory addresses = new address[](1);
        uint256[] memory integers = new uint256[](1);
        string[] memory strings = new string[](1);
        bool[] memory bools = new bool[](1);
        
        strings[0] = message;

        // Ensure lengths comply with bridge constraints
        require(addresses.length <= 5, "Addresses array length must be <= 5");
        require(integers.length <= 5, "Integers array length must be <= 5");
        require(strings.length <= 5, "Strings array length must be <= 5");
        require(bools.length <= 5, "Bools array length must be <= 5");

        // Prepare CrossChainData
        CrossChainData memory data = CrossChainData({
            addresses: addresses,
            integers: integers,
            strings: strings,
            bools: bools
        });

        // Call the bridge's outboundMessage
        bridge.outboundMessage{ value: msg.value }(
            address(this), 
            0x40A424C6e75eB7eCCE1A6Bc44F3b440d26FA7FD2, 
            data, 
            "ETH-Sepolia"
        );

        return true;
    }

The sendMessage function calls the outboundMessage function of the bridge contract and passes in several parameters:

  • address(this) is the address of the contract that is sending the message

  • 0x0000000000000000000000000000000000000000 is the address of the contract that the message is intended for (in this case, it's a placeholder)

  • addresses, integers, strings, bools are arrays that can be used to pass along data in the message.

  • "ETH-Rinkeby" is a string parameter specifying which chain the message is going to.

You can also pass more variables and customize the data that you're passing, depending on your use case.

Also please note that the contract should have enough balance to perform the transaction, otherwise the call will fail.

You've now implemented the ability to send messages to the bridge contract from your basic solidity contract. This will allow your contract to communicate with other chains. But it's not done yet, we need to listen to the incoming messages and handle them as desired. In the next step, you will add event listeners to the bridge contract to handle the incoming messages.

Step 4: Listen for incoming messages

In this step, we will add event listeners to the bridge contract to handle incoming messages.

The first thing you need to do is to define an event in the HelloWorld contract. This event will be triggered when a message is received by the contract.

Here's an example of how you can define the NewMessage event:

event NewMessage(string message);

Now, you need to add a function in the HelloWorld contract that will be responsible for handling the incoming messages. Here's an example of how you can define the portMessage function:

function portMessage(address[] memory addresses, uint256[] memory numbers, string[] memory strings, bool[] memory bools) public {
        require(addresses.length > 0, "Address array empty");
        require(strings.length > 0, "String array empty");

        // Set the message to the incoming one
        message = strings[0];
        emit NewMessage(strings[0]);
    }

The portMessage function accepts several parameters:

  • addresses is an array that can be used to pass along the address data in the message.

  • numbers is an array that can be used to pass along the numbers data in the message.

  • strings is an array that can be used to pass along the strings data in the message.

  • bools is an array that can be used to pass along the booleans data in the message.

This function includes a simple check to make sure that the addresses and strings arrays are not empty, and emits the NewMessage event passing the first string of the strings array as the parameter.

You can also add more validation and security to the function, for example, check whether the address data is coming from the correct source.

Finally, you can listen to this event in your HelloWorld contract and handle the received message however you want.

This function should be called by the bridge contract to pass the received message to the HelloWorld contract.

That's it! you have successfully set up a way for your contract to listen for and handle incoming messages across different chains. Please keep in mind that you need to adjust the logic in the portMessage function according to your needs.

Your final contract should look something like this:

pragma solidity ^0.8.0;

contract HelloWorld {
    string public message;
    event NewMessage(string message);

    // Bridge instance
    Bridge public bridge;

    // Set the bridge address upon deployment
    constructor(address bridgeAddress) {
        message = "Hello, World!";
        bridge = Bridge(bridgeAddress);
    }

    // Function to set a new message
    function setMessage(string calldata newMessage) external {
        message = newMessage;
        emit NewMessage(newMessage);
    }

    // Function to get the current message
    function getMessage() external view returns (string memory) {
        return message;
    }

    function portMessage(
        address[] memory addresses,
        uint256[] memory numbers,
        string[] memory strings,
        bool[] memory bools
    ) public {
        require(addresses.length > 0, "Address array empty");
        require(strings.length > 0, "String array empty");

        // Set the message to the incoming one
        message = strings[0];
        emit NewMessage(strings[0]);
    }

    function sendMessage() public payable returns (bool) {
        require(msg.value > 0, "Fee amount must be greater than 0");
        address[] memory addresses = new address[](1);
        uint256[] memory integers = new uint256[](1);
        string[] memory strings = new string[](1);
        bool[] memory bools = new bool[](1);

        strings[0] = message;

        // Ensure lengths comply with bridge constraints
        require(addresses.length <= 5, "Addresses array length must be <= 5");
        require(integers.length <= 5, "Integers array length must be <= 5");
        require(strings.length <= 5, "Strings array length must be <= 5");
        require(bools.length <= 5, "Bools array length must be <= 5");

        // Prepare CrossChainData
        CrossChainData memory data = CrossChainData({
            addresses: addresses,
            integers: integers,
            strings: strings,
            bools: bools
        });

        // Call the bridge's outboundMessage
        bridge.outboundMessage{value: msg.value}(
            address(this),
            0x40A424C6e75eB7eCCE1A6Bc44F3b440d26FA7FD2,
            data,
            "ETH-Sepolia"
        );

        return true;
    }
}

// CrossChainData struct as required by the bridge
    struct CrossChainData {
        address[] addresses;
        uint256[] integers;
        string[] strings;
        bool[] bools;
    }

    
// Define the Bridge interface
interface Bridge {
    function outboundMessage(
        address from,
        address to,
        CrossChainData calldata data,
        string calldata chain
    ) external payable;
}
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